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(11) | EP 0 133 902 A2 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Paper sizing process |
| (57) In the paper sizing in which synthetic cellulose-reactive sizing agents and calcium
carbonate as the filler are used, the aluminum polyhydroxychloride of formula:
wherein 0.96≤ x ≤6 and n = 2 x has an excellent retention effect. |
[0001] The present invention relates to a process for the sizing of paper, suitable for the sizing in the presence of a filler sensible to an acidic environnment, particularly calcium carbonate.
[0002] The paper sizing phase, consisting in the adhering to each other of the cellulosic fibers of the material forming the pulp by means of a sizing agent, may essentially take place according to two techniques:
- In acidic environnment (at a pH of between 4.5 and 6.5) wherein the sizing agents (which are generally derivatives of variously saponified resinic acids), must be precipitated to adhere to the fibres, by means of a suitable precipitating agent (aluminum sulphate or aluminum polychloride).
- In neutral enviromment (at a pH of between 6.5 and 7.5) wherein the sizing agents are of the synthetic cellulose-reactive type, namely which adhere to the fibers without the need of being precipitated and thus without requiring the use of a precipitating agent.
[0003] Examples of cellulose-reactive sizing agents comprise Aquapel, manufactured and sold from Hercules Inc., Basoplast of Badische Anilin and Soda Fabrik A.G. (BASF) and Tenoresin of Ciba-Geigy.
[0004] In both techniques the greater interaction between the sizing agent and the fibres takes place, owing to causes of chemical and physical nature, with the finest fibres and the less the fibres are dispersed, namely the more the fibres tend to be separated from the encompassing water, the better is the sizing.
[0005] By reducing the fibre dispersion, the amount of fibres and of sizing agent retained onto the wire during the sheet forming shall be greater; on the contrary a lesser amount of fibres and of sizing agent shall be lost with the effluent waters under the wire.
[0006] Thus, according to the processes to date known and used, whilst in the case of the resin sizing agents such a result is achieved with the precipitating agents, in the case of the synthetic cellulose-reactive sizing agents the dispersion is supplemented with properly dosed products or retention agents, such as polyamines, polyacrilamides, polyethylenamines and the like, these products being remarkably expensive.
[0007] It is also known that, depending on the end use of the paper, the fibre suspension is added with an inert filler, adapted to confer particular properties to the paper.
[0008] Among the most known and used fillers, kaolin and calcium carbonate can be cited. When the sizing agent is of conventional type, namely of resinic type, the combined use of kaolin and aluminum sulphate does not involve particular problems, except that the kaolin must be particularly pure, since possible impurities as calcium carbonate react with aluminum sulphate, whereby part of the precipitating agent is subtracted from the specific action as such.
[0009] In addition thereto an economical not negligible problem is to be taken also into consideration, since the kaolin having the desired purity has a remarkable cost. When the filler consists of calcium carbonate, the use of aluminum sulphate as the precipitating agent is practically inhibited since a relevant part of calcium carbonate is consumed, to form calcium sulphate, in order to neutralize the dispersion before the calcium carbonate may provide its action.
[0010] This problem has been solved and is the subject of a preceeding patent application of the same applicant, by using, in a sizing process with conventional resin sizing agents, aluminum polychloride and polyhydroxychloride as the precipitating agent concomitantly with the use of calcium carbonate as the filler, since it had been found that, contrarily to the chemically plausible forecasts, these precipitating agents operate in a neutral environnment (pH of about 7) in the fibre dispersion.
[0011] In the case of the sizing processes with synthetic cellulose-reactive sizing agents and in which calcium carbonate or another filler sensible to an acidic environnment is used, the recourse to the afore said synthetic retention agents has also been dictated until now from the above mentioned problems connected to the use of calcium carbonate.
[0012] Besides the above problems, another one occurs in the specific case of the fibre suspensions containing not negligible percentages of mechanical pulp, namely pulp obtained by mechanical grinding of the wood without chemical attack.
[0013] In fact, in that case, for the good operation of the continuos paper making machinery, essential importance has, besides the retention, also another operating parameter,
[0014] .namely the ready separation of the white water from the wire, this parameter being named freeness.
[0015] In fact, with the subject type of pulp, it is found that if the retention is high, the freeness is too much low and viceversa.
[0016] The use of aluminum salts different from the aluminum sulphate has been already proposed in the past.
[0017] For instance in the Canadian patent No. 759.363 the use if foreseen of a basic aluminum salt having the formula
wherein X is a monovalent anion, for instance chlorine, n is 1 to 20, particularly 4 to 10, m is less then 3n and the ration Al/X is of between 1.5 and 6, particularly of between 2 and 4.
[0018] The specific problem openly faced in this patent is that of the improvement of the paper sizing by avoiding the use of aluminum sulphate, and consequently both the specification and the claims are fully dedicated to the comparison with aluminum sulphate.
[0019] As regards the filler, reference is generically made to the fact that the starting pulp slurry may contain, if requested, the normal fillers without further detailed indication.
[0020] In turn the UK Patent Application No. 2010352 to Ginocchio et al discloses the use, in the paper sizing, of aluminum polyhydroxychloride having the formula:
as the precipitating agent for a sizing agent of resinic type (namely based on resin ),with the specific aim of eliminating the corrosion and bad odor problems related to the conventional use of aluminum sulphate.
[0021] Lastly the French Patent No. 2418297 discloses a paper sizing process having the purpose of improving the properties of the sized paper and the filler retention in the paper being manufactured.
[0022] To this end there is taught the use, as the precipitating agent, of a basic salt having the formula:
wherein M can be aluminum, X chloride, 3n-m 0 and the basicity, namely the % ratio m/3n, is of between 30% and 75%.
[0023] There is particularly preferred the presence in the stated salt of a polyvalent anion, for example the anion of a mineral acid such as sulphuric acid.
[0024] In the preamble the possible fillers are indicated, namely kaolin, calcium carbonate, talc, etc. without establishing any difference amongst them.
[0025] Both the specification and the examples, however, constantly refer to rosin based, i.e. resinic sizing agents, and to the use of a retention agent, such as starch, polyacrylamide and the like. It is thus confirmed the previous distinction, in this particular technology, between the processes using rosin sizing agent, i.e. colophony based sizing, and the processes with synthetic cellulose-reactive sizing agents.
[0026] In fact, in the latter case, as already mentioned, no precipitating agents were used and the fibre suspension was on the contrary added with agents, also of synthetic type, promoting the retention.
[0027] It has been now found and is the subject of the present invention that the aluminum polyhydroxychloride has an excellent retention action and at the same time permits calcium carbonate to be used as inert filler since the pH of the fibre dispersion is about 7.
[0028] Otherwise stated it has been found that the aluminum polyhydroxychloride normally used as precipitating agent in the sizing of paper with conventional resinic sizing agents, has, in the case of the synthetic cellulose-reactive sizing agents, an action substantially different but important as well in view of the results of the sizing process, namely a retention effect towards the fibres and the additives forming the pulp.
[0029] At the same time the addition of aluminum polyhydroxychloride permits the use of fillers sensible to acidic environnment, the use of which would be difficult if another well known and popular precipitating agent, used for resinic sizing agents, namely aluminum sulphate, would be employed. In the case of aluminum sulphate the experimental tests demonstrated that, in order to obtain a retention effect, satisfactory and comparable with those of the aluminum polyhydroxychloride, the needed amount of aluminum sulphate would give place to a pH of the fibre dispersion not higher than 6.4. Under these conditions of use calcium carbonate would raise difficulties and obviously the retention effect of the aluminum sulphate would be reduced.
[0030] The process according to the present invention is thus characterized in that to the aqueous dispersion of fibres, supplemented with a synthetic cellulose- reactive sizing agent and possibly with calcium carbonate as the filler, aluminum polyhydroxychloride having the formula:
is added.
[0031] According to the preferred embodiment the amount of aluminum polyhydroxychloride added to the fibre dispersion is such as to determine a retention, as measured as transparency, of about 40%
[0032] As already mentioned particular problems are met in the paper sizing starting from pulp slurry containing substantial amounts of mechanical pulp. It has been surprisingly found that in this case the use of aluminum polyhydroxy chloride of formula (4) permits acceptable values to be achieved both as regards the freeness and as regards, retention such as to guarantee a smooth operation of the continuos paper manufacturing machine.
[0033] The aluminum polyhydroxychloride useful in the process according to the invention is commercially available (Prodefloc SAB/18, sold by Prodeco Spa) , and is anyhow manufactured as described in the USP 3909439, which is herein referred to for further details.
[0034] The results of experimental tests, carried out with aluminum polyhydroxychloride (hereinafter indicated by the abbreviation PAC), in comparison with aluminum sulphate, are hereinafter reported.
[0036] The retention was evaluated by measuring the residual turbidity of the filtered water at Shopper-Riegler, by means of a transparent, overturned pyramid, bearing millimetric graduations.
[0037] The retention values were indicated as % of transparency of the drainage waters. As an optimum value, the retention giving place to a transparency percentage of at least 40 was taken.
EXAMPLE 1
[0038]
In table 1 the retention and the pH behaviour owing to the addition of increasing percentages of aluminum sulphate and of aluminum polyhydroxychloride (PAC) are reported.
[0039] As it can be seen, the retention value considered as optimum is achieved with a sulphate percentage of 16.2%, but for this amount the pH is lowered to 6.4.
[0040] It means that it is not possible to achieve a good retention with sulphate without simultaneously modify the buffering effect of carbonate with consumption thereof and CO 2 development.
[0041] As regards PAC, on the contrary to obtain a retention of 40% transparency an amount of 3.1 % PAC is needed.
[0042] At this percentage the pH value is 6.96 and is consistent with the carbonate buffered environnment.
[0043] In the tests carried out in a machine the pH of the cycle had to be maintained above 6.6 in order to avoid operation problems of the continuous machine and thus production drawbacks.
[0044]
With reference to the above mentioned problem of the fibre slurries containing mechanical pulp, comparison tests have been carried out using (a) the polyhydroxychloride of formula (4) according to the present invention, (b) the basic salt of the French patent No. 2418297, commercially available under the abbreviation WAC and (c) alum (namely aluminum sulphate).
[0045] The tests particularly related to the determination of retention and of freeness and to fibre slurries containing varying percentages of mechanical pulp.
EXAMPLE 2
Fibre slurry:
EXAMPLE 3
Fibre slurry:
[0048]
As it can be appreciated from the preceeding examples, PAC shows the best values of freeness and of retention in comparison both with alum and with WAC.
1. Paper sizing process, of the type in which an aqueous dispersion of cellulosic
fibres is added with a synthetic cellulose-reactive sizing agent and possibly with
a filler sensible to acidic enviromment, characterized in that said aqueous dispersion
is added with a retention agent consisting of aluminium polyhydroxychloride having
the formula:
wherein 0.965 x≤6 and n= 2x
wherein 0.965 x≤6 and n= 2x
2. A process according to claim 1, characterized in that said filler is calcium carbonate.
3. A process according to claim 1, characterized in that said aluminum polyhydroxychloride
is added in an amount such as to determine a retention corresponding to a transparency
of at least 40% when measured by the Schopper-Riegler method.
4. A process according to claim 1, characterized in that an aqueous dispersion of
fibres, containing 5 to 40% by weight of mechanical pulp, is treated.